1. Field of the Invention
The invention relates to a method and a device for carrier-frequency synchronization in the case of an offset quadrature-phase-modulated signal.
2. Related Technology
When transmitters and receivers are synchronized with one another in a transmission system, the clock-pulse and carrier signal are matched respectively with regard to phase position and frequency at the transmitter-end and receiver-end. The carrier-frequency synchronization to be considered below presupposes a received signal synchronized with regard to the clock-pulse signal.
DE 103 09 262 A1 describes a method for carrier-frequency synchronization of a signal with digital symbol sequences, in which the frequency and/or phase offset of the carrier signal is estimated from the demodulated received signal by means of maximum-likelihood estimation. The received signal containing the digital symbol sequence consists of complex rotary phasors associated with the individual sampling timing points, of which the discrete phases are dependent only upon the required frequency and/or phase offset of the carrier signal, and of which the discrete amplitudes are dependent only upon the digital symbol values of the demodulated received signal. The maximum-likelihood estimation of the required frequency and/or phase offset of the carrier signal is implemented by maximization of the likelihood function, which is formed from the sum of the real components of all time-discrete, complex rotary phasors of the received signal. The maximization of the likelihood function is implemented by rotating the complex rotary phasor of the received signal associated with each sampling timing point in the clockwise direction at the level of the required frequency and/or phase offset so that it is disposed on the real axis. In this manner, it is possible to obtain the required frequency and/or phase offset of the carrier signal by observing the extreme values of the likelihood function in each case separately for the frequency and/or phase offset.
In the context of DE 103 09 262 A1, the time-discrete received signal provides one complex rotary phasor at each sampling timing point, of which the phase value is dependent only upon the frequency and/or phase offset of the carrier signal, and of which the amplitude value is dependent upon the symbol value of the received signal sequence to be transmitted at the respective sampling timing point. A time-discrete received signal of this kind is based upon a comparatively simple modulation, for example, a conventional binary-phase phase sampling (BPSK). By contrast, if a more complex modulation method, especially an offset quadrature phase modulation (offset-QPSK-modulation) is used, considerable inter-symbol interference, which additionally disturbs the phases of the time-discrete complex rotary phasors, occurs in the received signal because of the squaring and also because of the phase displacement of the in-phase relative to the quadrature component at the level of half of the symbol period in the context of an offset QPSK modulation. Accordingly, the phases of the time-discrete complex rotary phasors of the received signal are not only dependent upon the frequency and/or phase offset of the carrier signal. In this case, the use of a maximum-likelihood estimation for the estimation of the required frequency and/or phase offset of the carrier signal in the sense of the method and the device known from the DE 103 09 262 A1 therefore fails to achieve the object.